I encountered this with another title and the cause was not the security software but a ROOTKIT that we found after the security software failure (just like your story.)
What is hard for everyone is they might get uppity or dig in their heels that McAfee caused the issue when there was something else amiss.
Hope this helps,
I recently downloaded a McAffee security update through my service provider who is BT. The computer required a restart to activate but would not complete the start up beyond showing my screen server then going through the restart over and over again. I managed to restart the machine by causing it to crash and starting up in safe mode which allowed me to remove the security software, I had tried the software start up several times without success. I have downloaded Microsofts essentials software and the start up remains the same as before with the Mcaffee system. Once sterted up the machine runs fine, I am now using avasts security but not keen on trying a restart, can anyone throw any further light onn the problem please???